Hybrid Electric Vehicles (HEV's) may buffer fuel energy by using the internal combustion engine to turn a generator. The electricity produced by the generator may be stored in a battery. HEV systems can also recover kinetic energy by using the momentum of the vehicle to turn the generator. The electricity produced also may be stored in the battery. Fuel is the predominate initial source of energy in an HEV system. Plug-in Electric Hybrid Vehicles (PHEVs) are an extension of existing HEV's with added energy flexibility. A PHEV utilizes a larger capacity battery pack than a standard HEV and the PHEV has two initial sources of energy; fuel as well as electricity from the electric utility grid.
An objective of a HEV control system may be to minimize energy operational costs and emissions without compromising the vehicle drivability and system constraints. Energy management control strategies may operate the HEV in electric drive mode in which the vehicle is operated with only the electric motor providing propulsion to maximize the battery power output. In a hybrid operation mode, the vehicle is propelled by both the engine and the electric motor.